Separate setting build environment and run environment in packages (#11115)

* Methods setting the environment now do it separately for build and run

Before this commit the `*_environment` methods were setting
modifications to both the build-time and run-time environment
simultaneously. This might cause issues as the two environments
inherently rely on different preconditions:

1. The build-time environment is set before building a package, thus
the package prefix doesn't exist and can't be inspected

2. The run-time environment instead is set assuming the target package
has been already installed

Here we split each of these functions into two: one setting the
build-time environment, one the run-time.

We also adopt a fallback strategy that inspects for old methods and
executes them as before, but prints a deprecation warning to tty. This
permits to port packages to use the new methods in a distributed way,
rather than having to modify all the packages at once.

* Added a test that fails if any package uses the old API

Marked the test xfail for now as we have a lot of packages in that
state.

* Added a test to check that a package modified by a PR is up to date

This test can be used any time we deprecate a method call to ensure
that during the first modification of the package we update also
the deprecated calls.

* Updated documentation

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